A gearshift in a semi-automatic change-speed transmission usually takes place in such manner that during a load reduction of the internal combustion engine the torque (motor torque) delivered by or taken up from the internal combustion engine is decreased and a friction clutch is opened, whereby the transmission becomes largely free from load. Then, the currently engaged gear carrying the load before the gearshift is disengaged, and the target gear to be engaged is synchronized and then engaged. To synchronize the target gear the rotational speed of the transmission input shaft is reduced for an upshift and increased for a downshift, so that the two clutch halves of the gear clutch of the target gears are rotating at much the same speed.
In the case of synchronized gear clutches, shift-related braking and acceleration of the transmission input shaft takes place by virtue of associated friction rings which are pressed together by the displacement force of the gear control element concerned. With unsynchronized gear clutches, shift-related braking and acceleration of the transmission input shaft can be effected by auxiliary devices, for example by means of a transmission brake in driving connection with the transmission input shaft, or an electric machine in driving connection with the transmission input shaft, or by the internal combustion engine if the friction clutch is at least partially closed. Once the target gear has been engaged the friction clutch is completely closed again and the motor torque of the internal combustion engine is increased again.
Although a gearshift controlled in such a manner takes place relatively comfortably and with low wear, it takes a comparatively long time and entails a relatively long interruption of the traction or thrust force. Accordingly, several methods for controlling shifts in semi-automatic transmissions have been proposed, in which a gearshift takes place in combination with suitable control of the internal combustion engine while the friction clutch is at least partially and/or temporarily closed.
For example, DE 199 54 605 A1 describes a method for controlling a semi-automatic transmission, in which a gearshift during emergency operation takes place with the friction clutch closed. In this case it is provided that the torque delivered by or taken from the internal combustion engine is reduced down to approximately zero and the currently engaged gear is then disengaged while largely free from load. Thereafter the transmission input shaft is decelerated or accelerated to the synchronous speed of the target gear by means of the internal combustion engine, and the target gear is then engaged.
From DE 101 22 158 A1 a method for controlling a semi-automatic transmission is known, in which a traction upshift takes place up to the slipping limit of an open friction clutch. After the partial opening of the friction clutch, the motor torque is first reduced, then the current gear is disengaged, and thereafter the target gear is engaged without synchronization. After the target gear has been engaged the speed of the drive motor is made equal to the rotational speed of the transmission input shaft by the motor control system and/or by closing the friction clutch, before the motor torque is increased again. In a transmission suitable for this shifting method, except for those of the starting gears the gear clutches can be in the form of unsynchronized claw clutches.
EP 0 676 566 B1 describes a method for controlling an internal combustion engine during an upshift of a semi-automatic transmission with the friction clutch closed. According to this, at the beginning of the upshift the internal combustion engine is decoupled by control means from the accelerator pedal and by manipulation of the fixed injection quantity a motor speed fluctuation is induced, which enables the current gear to be disengaged while largely free from load. Once the current gear has been disengaged the speed of the internal combustion engine is brought to the synchronous speed of the target gear by reducing the fuel injection quantity, i.e. the transmission input shaft is synchronized, so that the target gear can be engaged. Once the target gear has been engaged the motor control system is again coupled to the accelerator pedal, i.e. to produce a corresponding motor torque the injection quantity of the internal combustion engine is increased. The transmission for implementing this method is for example a group transmission with a multi-stage, unsynchronized main transmission and a two-stage, synchronized range group connected downstream from it.
In a method according to DE 102 49 951 A1 for controlling a drive-train that comprises an internal combustion engine, an automated friction clutch and a semi-automatic transmission it is provided that before any gearshift, as a function of measured operating parameters and with reference to set testing rules it is checked whether the shifting process intended can be carried out with a closed friction clutch. If it can, the gearshift is carried out without opening the friction clutch. Otherwise, the gearshift takes place in combination with opening the friction clutch. The transmission for using this control process is for example in the form of a group transmission comprising a multi-stage main transmission and a two-stage splitter group connected upstream from it.
DE 10 2006 054 277 A1 describes a method for carrying out a gearshift in a preferably unsynchronized semi-automatic transmission, in which the current gear is disengaged with the friction clutch closed, and the decision whether the target gear should also be engaged with the friction clutch closed or open during the gearshift sequence is made as a function of operating parameters. If the disengagement of the current gear is not completed within a specified time interval, the shifting mode is changed to one with the friction clutch open. To accelerate the opening of the friction clutch that may be necessary, the clutch is opened to approximately its slipping limit already before the decision about the shifting mode.
Finally, from DE 10 2007 010 829 A1 a method is known for controlling a semi-automatic group transmission with a multi-stage main transmission, a two-stage splitter group connected upstream, and a two-stage range group connected downstream from it. In a range shift in which a change of the gear ratio takes place at least in the main transmission and in the range group, the target gear ratio in the range group is synchronized by partially closing the friction clutch after the friction clutch has previously been opened, the range group shifted to neutral, the main transmission shifted, and in parallel with this the internal combustion engine has been brought to the synchronous speed of the target gear.
Such control methods have the disadvantage that the load reduction of an internal combustion engine at the beginning of a shifting process, the load increase of an internal combustion engine at the end of a shifting process, and the changing of the speed of an internal combustion engine to the synchronous speed of the target gear, take place relatively slowly because of the high moment of inertia of the internal combustion engine and its indirect control by means of the ignition and/or the fuel injection, so the time taken for the shifting process concerned is made longer. Furthermore, due to the control sequences of the internal combustion engine and the friction clutch during a shifting process torque peaks and speed surges can occur, which adversely affect the shifting or driving comfort of the motor vehicle concerned.